1. Field of the Invention
This invention relates to a combustion apparatus having a heat-recirculating function which preheats a fuel-air mixture using the heat-recirculation from high temperature product gas, in particular to a combustion apparatus employing a surface combustion burner as the combustion zone (burner) thereof.
2. Description of the Related Art
Many types of plane exothermic surface combustion burners have been developed and commercialized as a heat source for industrial use as well as for household use because they easily generate emissive power with low pollutant emission by adopting a porous burner component in a shape of flat plate, curved plate, or block provided with many gas passage pores and by combusting the fuel-air mixture, passed through the porous burner component, at the downstream surface of the component to form a uniform plane red-heat condition stably existing thereon.
Since the combustion apparatus using a surface combustion burner has to form a combustion zone in the porous burner component near the surface thereof to make the plane red heat combustion zone a high radiation condition, the combustion apparatus with a surface combustion burner has the limitation that the fuel-air mixture cannot be ejected from the porous burner component at a higher speed than the burning velocity of the mixture thereof.
The equivalence ratio of flammable fuel-air mixture (the value of fuel-air ratio divided by theoretical fuel-air ratio) is in a range of 0.9 to 1.1, in order to gain higher emissive power. The adiabatic flame temperature within that range considerably exceeds the durable limit of, for example, a metallic sintered fiber mat used as the porous burner component. As a result, most of the supplied heat has to be emitted as the high temperature exhaust gas, and the radiation efficiency is only 25% (for equivalence ratio of 1) at the most.
Conventional heat-recirculating combustion apparatuses of heat-recirculating type have been introduced to reduce the heat supply by preheating the fuel-air mixture with the recovered waste heat of combustion gas and to substantially improve the radiation efficiency. These conventional heat-recirculating combustion apparatuses of heat-recirculating type are either of the type which provide heat exchange via a separation wall located between the fuel-air mixture supply passage and the combustion gas exhaust passage, or of the type which is provided with a specially designed heat exchanger or heat accumulator. Regarding the former type which provides heat exchange via a separation wall located between the fuel-air mixture supply passage and the combustion gas exhaust passage, insufficient heat transfer area of the separation wail results in an insufficient effect, so the size of the apparatus has to be enlarged. As for the latter type which is provided with a specially designed heat exchanger or heat accumulator, the added equipment complicates the structure of apparatus and enlarges the apparatus size, and furthermore the efficiency of heat exchanger and heat accumulator needs improvement.